A query about Black Holes

[Farsight]

There's something I'm puzzling about regarding black holes:

 

I think it's reasonable to assume that black holes exist. But we know from GPS and other evidence that clocks run slower here on earth than they do up in space. It's not an optical illusion or some kind of observer effect, it's something real, and it's down to gravity. This means time "runs slower" as you near an event horizon, whereupon it stops. Which suggests to me that collapsing stars are collapsing so slowly as far as our time experience is concerned, that they haven't finished collapsing yet. This means there can't be any actual singularities, because as far as we're concerned, the collapse takes an infinite length of time. And if Hawking Radiation is true, the black hole will evaporate before the collapse is complete.

 

Can anybody clear this up for me?

[jackson33]

black holes are a little complicated and not much is known. in the last few weeks, two interesting points have come up....1) in the process of implosion the the star rotation speed up dramatically. NASA observations shows three ranging from 300 to 1000 times per second. 2) gravity is thought to be about the same as any star of equal size..

 

giant stars that have lost there fuels or innards, burn out, collapse and form this unit and evaporate. the process may take millions of years but the process should be overlapping events. that is one process is ending another starting up. there is nothing i know to suggest the final collapse and evaporations could not both be in process.

 

this is opinion; i have long felt gravity is not the cause of what we see in BHT.

the process of matter imploding should create a vacuum that other matter should try to fill (suction) and this matter in movement would easily continue that direction and be involved in the BH theory. not necessarily that gravity is so strong for any unusual event.

[[Tycho?]]

There's something I'm puzzling about regarding black holes:

 

I think it's reasonable to assume that black holes exist. But we know from GPS and other evidence that clocks run slower here on earth than they do up in space. It's not an optical illusion or some kind of observer effect, it's something real, and it's down to gravity. This means time "runs slower" as you near an event horizon, whereupon it stops. Which suggests to me that collapsing stars are collapsing so slowly as far as our time experience is concerned, that they haven't finished collapsing yet. This means there can't be any actual singularities, because as far as we're concerned, the collapse takes an infinite length of time. And if Hawking Radiation is true, the black hole will evaporate before the collapse is complete.

 

Can anybody clear this up for me?

 

I can add a few comments, other things I dont know too much about. But ignore jackson33, he has no idea what he is talking about.

 

I dont think time "stops" at the event horizon. Time dilation is related (in some way) to the strength of the gravitational field. The event horizon is just the limit where light can't quite escape. But gravity there is still finite, and so when talking about time dilation effects, I dont think the event horizon is different from anywhere else (except the singularity of course).

 

As another note, Hawking radiation will not cause most black holes to evaporate. It does mean that black holes radiate energy, but the cosmic microwave background radiation alone is enough to counteract this, so a steller black hole will not have a net loss of mass.

 

But for the meat of your point though, I dont think I can help you. Ive heard descriptions similar to yours and I'm none too sure what to make of them. Martin might be able to help you here, although I havn't seen him around for a while.

[Ragib]

You are under the assumption that the time stops from our frame of reference, even before the singularity forms. And also, time slows down because information, or light, takes longer to travel to us. In the frame of reference of the singularity however, time is going normal. In its own frame it can form. We may see it finally finish forming much later than it actually does, but eventually will.

[Spyman]

Farsight,

 

Here are two threads of similar topic which I think basically agree with you:

http://www.scienceforums.net/forum/showthread.php?t=8289

http://www.scienceforums.net/forum/showthread.php?t=8762

 

A thread about Hawking radiation including the evaporation rate:

http://www.scienceforums.net/forum/showthread.php?t=9500

 

A thread about Loop quantum gravity which may remove the singularity:

http://www.scienceforums.net/forum/showthread.php?t=21469

 

There are problably several others too, but I'm to lazy to search them all...

(You may hunt them down yourself if you feel the need for more.)

[Farsight]

Thanks guys.

 

Special thanks Spyman, I'll check them out.

 

Ragib, the problem is this: Yes, in their frame WHAM the collapse is done. But right now in what we call 2006, they haven't even got to the W of WHAM. And they never, ever, will. This means something has got to give. We either kick out the singularity because it's never going to happen. Or we kick out time. Personally I prefer to kick out time.

[rocket art]

Time gets 'blurred'.

[Janus]

;313265']

 

I dont think time "stops" at the event horizon. Time dilation is related (in some way) to the strength of the gravitational field. The event horizon is just the limit where light can't quite escape. But gravity there is still finite' date=' and so when talking about time dilation effects, I dont think the event horizon is different from anywhere else (except the singularity of course).

 

[/quote'] Time dilation is not related to local gravitational field strength but rather to difference in gravitational potential.

 

As far as time dilation at the event horizon goes:

 

For a simple case, the time dilation is given by

 

[math]t_f = \frac{t_0}{\sqrt{1- \frac{2GM}{Rc^2}}}[/math]

 

The Schwartzchild radius (the radius of the Event Horizon) of a blackhole is given by

 

[math]R = \frac{2GM}{c^2}[/math]

 

If we substitute this R into the time dilation equation it reduces to

 

[math]t_f = \frac{t_0}{0}[/math]

 

IOW, time stops at the event horizon according to a distant observer.

 

One thing to note istthat in the time dilation formula given, our observer is considered to be an infinite distance from the Black hole.

[Farsight]

Thanks Janus.

[Ragib]

But wouldn't the time only start going infinity slow, as from our point of view, once the singularity is already formed, or close to forming, since before that point the gravitational potential is still not that much?

 

Or another argument, Sure in our frame of reference we would never see a singularity since the information could not escape to us. But we would see effects around it.

[swansont]

Time dilation is not related to local gravitational field strength but rather to difference in gravitational potential.

 

As far as time dilation at the event horizon goes:

 

For a simple case, the time dilation is given by

 

[math]t_f = \frac{t_0}{\sqrt{1- \frac{2GM}{Rc^2}}}[/math]

 

The Schwartzchild radius (the radius of the Event Horizon) of a blackhole is given by

 

[math]R = \frac{2GM}{c^2}[/math]

 

If we substitute this R into the time dilation equation it reduces to

 

[math]t_f = \frac{t_0}{0}[/math]

 

IOW, time stops at the event horizon according to a distant observer.

 

One thing to note istthat in the time dilation formula given, our observer is considered to be an infinite distance from the Black hole.

 

 

The thing I have never understood is the step from the time dilation diverging to motion stopping. One doesn't measure motion using the clock of the observed particle, it uses the local clock. If I accelerate a muon to a very high speed, I notice its decay taking longer, according to my clock, but the thing still moves. I can see that the particle never sees itself fall into the black hole, if the time dilation indeed diverges, since it would observe the rest of the universe's time outside the Schwarzschild radius moving infinitely fast.

[timo]

Tom: Motion stopping in this case indeed means the ratio of coordinate distance to coordinate time dr/dt, not the ratio of coordinate distance to eigentime dr/[math]d\tau[/math]. The particle witnesses itself as falling into the black hole within finite (eigen-)time. I´ve attached a plot below. It only shows the integration of the equation of motion till the event horizont (i.e. the particle doesn´t pass the event horizont) because the calculation used the same "improper coordinates" (Schwarzschild coordinates) as Janus' post above and those coordiantes have a singularity at the event horizont. But it -hopefully clearly- shows that the particle seems to stop in coordinate time, while it seems to simply fall through the event horizont if you describe the fall in eigentime.

Attached image: http://www.scienceforums.net/forum/attachment.php?attachmentid=1462&stc=1&d=1167412943

 

 

ON THE ORIGINAL TOPIC:

The apparent stopping of a particle is usually calculated for an already-formed black hole. Most importantly, the solution you are usually given is the solution for the spacetime geometry outside the mass distribution. During formation of a black hole, the conditions that all mass is inside the event horizont is obviously not met (not for the scope of your question, at least). I am not sure if the spacetime geometry inside the mass distribution would also predict this "particles seem to stop at the event horizont"-effect. Quite on the contrary, I would be pretty surprised if a stone you´d throw into a hole through earth would suddenly come to a stop near the center because it reached the event horizont of earth.